HDMI is an interface through which video signal output apparatuses such as digital video cameras, video recorders, tuners, and so on, can be connected to video control apparatuses such as television receivers, and controlled using a single cable. HDMI stands for High-Definition Multimedia Interface.
HDMI is based on the conventional DVI (Digital Visual Interface) standard, and HDMI is capable of transmitting wideband video and audio signals, and has defined functions such as copyright protection, color difference transmission, and so on for use in household audio-visual electronics. Because HDMI is capable of implementing the stated functions simply by connecting devices with a single cable, it is advantageous for users in that it eliminates the cumbersome need to connect multiple cables. The increase in video signal output apparatuses capable of HDMI output has also led to an increase in video control apparatuses provided with multiple HDMI ports.
HDMI transmits information between connected devices using the TMDS, CEC, and DDC channels. TDMS stands for Transition Minimized Differential Signaling. CEC stands for Consumer Electronics Control. Finally, DDC stands for Display Data Channel.
The TMDS channel transmits video data, audio data, and auxiliary data.
The CEC channel transmits device control signals. The video control apparatus and video signal output apparatus can control one another, through communication using CEC commands (called “CEC communication” hereinafter), using EDID, which shall be mentioned later. For example, a video control apparatus can implement a function for automatically switching its input to the terminal to which a video signal output apparatus is connected in response to a playback operation performed by the video signal output apparatus.
EDID transmission and HDCP-based authentication are performed over the DDC. EDID stands for Extended Display Identification Data. HDCP, meanwhile, stands for High-bandwidth Digital Content Protection.
A video signal output apparatus obtains the display capabilities, audio output capabilities, and so on of a video control apparatus, as well as a physical address expressing its own connection location, based on the EDID held by that video control apparatus. The EDID obtainment operations commence when an HPD (Hot Plug Detect) signal of HDMI in the video control apparatus switches from “off” to “on”.
HDCP is a copyright protection technique that encrypts digital data such as video data, audio data, and so on sent from a video signal output apparatus to a video control apparatus, thereby preventing the unauthorized copying of content. When authentication has not been established between a video signal output apparatus and a video control apparatus, the video signal output apparatus immediately stops sending video data and audio data to the video control apparatus, and the video control apparatus also stops decrypting the received video data and audio data.
Here, a video control apparatus provided with multiple HDMI ports shall be considered. In order to enable inter-device control through the stated CEC communication, it is necessary for the video control apparatus to send the EDID to each of the video signal output apparatuses in advance and confirm the physical addresses of those video signal output apparatuses. Accordingly, for each of its HDMI ports, the video control apparatus includes EDID ROMs that store the EDID information.
Furthermore, the video control apparatus stores authentication information used for HDCP authentication in an HDCP ROM. This authentication information is used to decrypt the video data and audio data transmitted from the video signal output apparatus to the video control apparatus. Therefore, the video control apparatus should have a number of HDCP ROMs corresponding to the number of HDMI ports capable of simultaneous display on a single screen. In other words, generally speaking, while the video control apparatus is provided with an EDID ROM for each of its HDMI ports, it is equipped with fewer HDCP ROMs than the number of HDMI ports.
When DDC communication is carried out between the video signal output apparatus and the video control apparatus, if there are fewer HDCP ROMs than there are EDID ROMs, a situation arises in which the video signal output apparatus can access the EDID ROMs but cannot access the HDCP ROMs.
In such a case, no response can be obtained for an HDCP ROM access request, resulting in an undefined state in the DDC line. If the video control apparatus makes an input switch while the DDC line is in an undefined state, the DCC access cannot be restored to its normal state, and thus video signals, audio signals, and so on cannot be output normally. In addition, there are, for example, cases where the video signal output apparatus cannot return to its normal state from a state in which it is standing by for an HDCP ROM access request.
Accordingly, the video control apparatus temporarily puts the HPD signal into an “off” state when each HDMI port is selected and switched to, returning the HPD signal to the “on” state after the passage of a certain amount of time. This resets the DDC signal, and resuming communication thereafter solves the problem of the DDC line entering an undefined state.
Japanese Patent Laid-Open No. 2007-78980 discloses a technique related to DDC communication. According to Japanese Patent Laid-Open No. 2007-78980, when video data and audio data not described in the EDID is transmitted by the video signal output apparatus due to a DDC communication failure etc., the video control apparatus once again switches the HPD signal off and then on, thereby resuming the DDC communication. Resuming the DDC communication causes the video signal output apparatus to obtain the EDID anew and output normal video data and audio data as a result.
However, an undefined state occurs in DDC communication only when a video signal output apparatus that performs HDCP authentication is connected. For example, video signal output apparatuses that output a user's private content, such as a consumer digital video camera, does not require HDCP authentication to be performed. For this reason, DDC communication does not experience an undefined state, and thus processing such as that described above, where switching the HPD signal off and then on again to reset DDC communication and then resume DDC communication, is not necessary.
In other words, for devices that do not require HDCP authentication, it is not necessary to perform processing for turning the HPD signal off and then on again and resuming DDC communication each time an HDMI port is selected and switched to, and thus there has conventionally been the problem that an amount of time equivalent to the switching processing is required. This processing sometimes requires, for example, several seconds.
The technique disclosed in Japanese Patent Laid-Open No. 2007-78980 relates to resuming DDC communication after video data and audio data have been input, but does not discuss operations and processing time when switching inputs, and thus cannot solve this problem.
It should be noted that this problem is not limited to the HDMI standard, and may occur in other video control systems in which the communication line experiences an undefined state when the video control apparatus accesses an encryption key for the purpose of copyright protection.